1. Field of the Invention
Broadly speaking, this invention relates to seismic surveillance systems. More particularly, in a preferred embodiment, this invention relates to a seismic surveillance system in which the seismic transducers and receivers are coupled to the earth by means of selected, naturally growing vegetation found in the area to be surveilled.
2. Discussion of the Prior Art
Experimental seismic surveillance systems for detecting static and dynamic objects on or below the surface of the soil are known. Essentially, these experimental systems established a "seismic fence" for intrusion detection. Intruding vehicles or personnel passing between the seismic transducers which set up the "fence" were detected as a disturbance in a delicately balanced seismic loop.
A microseismic surveillance system can, in general, employ two methods of seismic excitation: (1) a transient impulse; or (2) a continuous seismic wave (CSW).
Although the transient impulse method could, in theory, be employed, it has not proved to be really practical, for many reasons, including the high cost and unreliability of the seismic impulse generators and the exceedingly complex nature of the return signals which are generated by the transient excitation.
Unfortunately, great difficulty has been experienced in coupling CSW transducers to the earth. If the transducer is merely placed on the earth, a very inefficient coupling is achieved and the area of contact is, of course, limited by the geography of the transducer. Further, in wet, marshy terrain, such as is frequently found in the jungle where seismic surveillance is of greatest importance, prior art transducers merely wallow in the mud and couple practically no seismic signals into the ground.